We are studying the properties and structure of the outer membrane of E. coli. Mutants showing increased permeability to a large number of hydrophobic compounds include a locus in which the only apparent change is loss of lipid A phosphate, suggesting that lipid A is responsible for hydrophobic impermeability of this membrane. We have also devised a gel electrophoresis method that shows far more heterogeneity in lipopolysaccharide from a single strain than heretofore detected. In other experiments we find that the small amount of protein that copurifies with lipopolysaccharide in butanol "endotoxin" preparations is all outer membrane protein, which is of interest because of the lymphocyte mitogenicity of this protein. In studies with macrophage cell lines we have defined by gel electrophoresis several external membrane proteins that are reduced by borotritide after sialic acid oxidation, and that completely lose their label on sialidase treatment. These will be used as markers in studies of membrane dynamics. In other experiments, mutants altered in cell surface components are being isolated.